Field of the Invention
The invention relates to a method for cooling the coolant of a gas turbine of a gas and steam turbine system having a first evaporator loop that includes a steam drum. The invention also pertains to an apparatus for cooling the coolant of a gas turbine of a gas and steam turbine system, for performing such a method.
Gas and steam turbine systems are typically used to generate electrical energy. Operating medium carried in an evaporator loop of the steam turbine system, typically a water-water/steam mixture, is evaporated in an evaporator, and the steam thus produced is used for generating energy.
In an evaporator loop of the natural circulation type, partial evaporation of the operating medium supplied is effected in the evaporator, and a circulation of the operating medium is assured by means of pressure differences arising in the evaporation process and/or by the geodetic disposition of the evaporator and the steam drum. Alternatively, the evaporator loop may also involve forced circulation, in which the circulation of the operating medium is assured by a recirculating pump, and the operating medium again evaporates at least partially in the evaporator. Both in natural and in forced circulation, the water-water/steam mixture from the evaporator is delivered to a steam drum connected into the evaporator loop. In the steam drum, water and steam are separated, and from the steam drum the water is redelivered to the evaporator.
Complete evaporation with ensuing partial superheating of the operating medium in one pass through the evaporator is effected where the evaporator loop involves forced once-through passage, in which the passage of the operating medium is again assured by means of pumps.
Regardless of whether natural circulation, forced circulation or the forced once-through principle is used in the steam turbine system, the goal--in order to increase the capacity of the gas turbine and hence to attain the highest possible efficiency of such a gas and steam turbine system--is an especially high temperature of the propellant gas at the inlet of the gas turbine, for example of from 1000.degree. to 1200.degree. C.
This kind of high turbine inlet temperature, however, entails material problems, particularly with respect to the heat resistance of the turbine blades.
Increasing the turbine inlet temperature can be allowed whenever the turbine blades are cooled enough that they always have a temperature below the allowable material temperature. To that end, it is known from European Patent EP 0 379 880 to divert air, compressed in a compressor assigned to the gas turbine, and to cool this air, acting as a coolant, before it enters the gas turbine. The heat extracted from the cooling air in the cooling process can be used for instance as heat of evaporation and can be used to drive the steam turbine. A flash evaporator with a recirculating pump and an expansion vessel is then used as the evaporator system. Water is drawn from the steam drum of the evaporator loop; this water is heated by heat exchange with the cooling air acting as coolant for the gas turbine and is then evaporated by expansion in the expansion vessel. The steam thus produced is supplied to the steam turbine.
Such an apparatus for cooling the cooling air of a gas turbine is designed for a particular temperature difference between the water drawn from the steam drum and the cooling air. For example, if because of peak-load operation or the withdrawal of process steam the process in the evaporator loop and hence the water temperature in the steam drum rise, then the temperature of the cooling air of the gas turbine rises as well. Hence cooling of the cooling air to the temperature necessary for cooling the gas turbine is practically no longer assured, which can cause failure of the gas turbine.
Gas turbines whose coolant is recoolable in a heat exchanger, wherein the temperature of the coolant is variable, are known from U.S. Pat. No. 4,891,937 (corresp. EP 0 106 313) and from U.S. Pat. No. 5,255,505 (corresp. GB 226 45 39).